The present invention relates to a high frequency variable gain amplifier device and a wireless communication terminal suited to use in, for example, cordless telephones and cellular telephones, etc.
With wireless communications terminals such as, for example, cellular telephones, when the distance to an opposing station is small, transmission power control is carried out with respect to the transmission in order to economize on transmission power usage, prevent saturation of high frequency amplifiers of an opposing station and reduce interference with other stations. In particular, in a Code Division Multiple Access (hereinafter referred to as "CDMA") method that is one cellular multi-connection method that has come to the forefront in recent times, highly precise transmission power control over a wide control band has been carried out to maintain line capacity.
On the other hand, at a receiver circuit, gain control is carried out in order to suppress fluctuations in the level of the received signal due to the influence of changes in the distance to an opposing station or faxing and to prevent saturation of the frequency controlled amplifier due to high level input signals.
An example will now be described with reference to FIG. 1 of the configuration of a cellular telephone taken as an example of a wireless receiving device.
In FIG. 1, an antenna 11 for transmitting and receiving signals is connected to a port 12a of an antenna sharing circuit 12. This antenna sharing circuit 12 is equipped with transmission side and receiving side ports 12t and 12r, with bandpass filters (not shown in the drawings) of prescribed characteristics being connected across the antenna and transmission side ports 12a and 12t and the antenna side and receiving side ports 12a and 12r.
A high frequency signal from the receiving port 12r or the antenna sharing circuit 12 is supplied to a mixer 22 via a low noise high frequency amplifier 21 of a receiver 20. A local oscillating signal from a local oscillator 23 is supplied to the mixer 22. The high frequency signal from the low noise high frequency amplifier 21 is then converted to an intermediate frequency signal. This intermediate frequency signal is supplied to a demodulator 25 and a received power detector 26 via an intermediate frequency amplifier 24.
The output of the received power detector 26 is provided as negative feedback to the intermediate frequency amplifier 24 and the low noise high frequency amplifier 21 so that the gain of these circuits is automatically controlled. The output of the demodulator 25 is supplied to a baseband signal processor 31 where it undergoes prescribed signal processing, and the received information is played back as audio signals etc.
The played back received information includes information indicating the power of the transmission from, for example, a base station and this indication information is taken out by a microcomputer 30.
Transmitted information such as audio signals etc. is subjected to prescribed signal processing at the baseband signal processor 31 and an output signal of the baseband signal processor 31 is supplied to a modulator 41 of a transmitter 40. The output of the modulator 41 is supplied to a mixer 43 via an intermediate frequency amplifier 42.
A local oscillating signal from the local oscillator 23 is supplied to the mixer 43. The intermediate frequency signal from the intermediate frequency amplifier 42 is converted to a high frequency signal and supplied to the transmission port 12t of the antenna sharing circuit 12 via a driving amplifier 44 and a high frequency power amplifier 45.
A transmission power control signal from a transmission power controller 46 is supplied to the intermediate frequency amplifier 42, driving amplifier 44 and the high frequency power amplifier 45 so as to control the gain of these circuits. This transmission power control signal is generated based on received signal power detection information from the received power detector 26 and transmission power indication information from the microcomputer 30.
In the above, a configuration is described for transmission power control where control of direct current power supplied from a power supply to this high frequency power amplifier is exerted in response to transmission power control information in such a manner as to raise efficiency of the high frequency power amplifier at the time of low transmission output.
On the other hand, with the high frequency amplifier of this receiving circuit, the configuration also has to be such that the desired receiving characteristics are also obtained for high level input signals and the direct current power supplied from the power supply to this high frequency amplifier when the input signal is of a low level is therefore reduced by controlling this power in response to the level of the received signal.
Portable communication terminals generally continue in a waiting to receive state for long periods of time. If savings can be made in the current consumption of the high frequency amplifier of the receiving side for this period, substantial saving in the power consumed can be made because this usage tends to be for long periods even though this amount is small compared with the amount saved for the high frequency power amplifier of the transmission side.
With related high frequency variable gain amplifiers, drain current of either drain is reduced by, for example, controlling the gate bias voltage or drain bias voltage of field effect transistors comprising amplifiers in response to the input level (transmission power control information at the transmission and received signal level at the receiving circuit) of the amplifiers. The gain of the amplifiers is therefore made to change and the consumed power is reduced.
However, in the related technology it is necessary to adjust the control voltage value and range due to variations in the semiconductor process. It is therefore difficult to obtain large gain control widths that are stable.
In order to resolve these problems, in Japanese Patent Application No. Hei. 7-248697 (applied for on the Aug. 31, 1995) a variable gain amplifier capable of obtaining a stable large gain control width and capable of achieving reductions in the power consumed is provided having a plurality of transmission paths of different gains. One of these paths is switched over to and selected by a high frequency change-over switching and the power supplies for amplifiers for other transmission paths are turned off.
FIG. 2 shows an example of the variable gain amplifier disclosed above. At a variable gain amplifier 50 of this example, there is a transmission path including a high frequency amplifier 51 of a prescribed gain G51 taking, for example, field effect transistors (referred to as FETs in this specification) as active elements and a transmission path of a through line 52 of a gain of 0 dB. These two transmission paths are switched between by high frequency change-over switch circuits 53 and 54. A high frequency signal from the input terminal Ti is then taken from an output terminal To via one of the transmission paths in response to the change-over states of the high frequency change-over switch circuits 53 and 54.
Further, the power supply voltage Vdd of the high frequency amplifier 51 is supplied via a feed switch 55.
In the example provided, the high frequency change-over switch circuits 53 and 54 and the feed switch 55 are controlled so as to be operated together by a controller 60 in response to the transmission power control information or the received signal level described above. A high frequency signal is then supplied to the through line 52 and when the high frequency amplifier 51 is not operating, the feed to the high frequency amplifier 51 is halted by the feed switch 55 so that unnecessary power consumption by the high frequency amplifier 51 is avoided.
In this case, the controller 60 controls the timing of the high frequency change-over switch circuits 53, 54 and the feed switch 55 in such a manner as to avoid the output terminal To not having a signal. This is because it is not desirable for output terminal To to have no signal when high frequency change-over switch circuits 53 and 54 are changed over after first switching over the feed switch 55 to the side of terminal n when the transmission path of the through line 52 of a gain of 0 dB is changed over to.
The high frequency change-over switch circuits 53 and 54 in the example in FIG. 2 are each configured of four FETs of FET Qa to Qd, as shown in FIG. 3. Here, the source and drains of the first and second FETs Qa and Qb are inserted in series across a common terminal Tc and the first and second terminals Ta and Tb, respectively. The source and drains of the third and fourth FETs Qc and Qd are connected across the common terminal Tc and ground.
Then, for example, when a d.c. voltage [-Vg] for control use is supplied to the gates of FETs Qa and Qd and a d.c. voltage [0]V for control use is supplied to the gates of FET Qb and FET Qc, the first and third FETs Qa and Qc go "ON" and the second and fourth FETs Qb and Qd go "OFF" so that a signal transmission path is formed across the common terminal Tc and the second terminal Tb. Tc and the second terminal Tb.
Further, by making the control voltages supplied to the gates of each of the FETs Qa to Qd the opposite of that above, the "ON" and "OFF" states of the FETs Qa to Qd are inverted and a signal transmission path is formed across the common terminal Tc and the first terminal Ta.
In the example of FIG. 2, control of the timing of switching over the high frequency change-over switch circuits 53, 54 and the feed switch 55 is troublesome. Further, insertion loss occurs for the high frequency change-over switch circuits 53 and 54 (Ls53+Ls54) due to FET Qa and Qb of the FETs Qa to Qd comprising the high frequency change-over switch circuits 53 and 54 and there is therefore the problem that the gain G51 of the high frequency amplifier 51 is reduced by this portion.
Moreover, as the high frequency change-over switch circuits 53 and 54 are comprised of four FETs Qa to Qd, respectively, the scale of the circuit becomes large and the cost expensive, with this being a particular problem with regards to the strict demands of compactness at a low price placed on portable communications terminals.
In order to take into account the above problems, it is an object of the present invention to provide a high frequency variable gain amplifier device and wireless communications terminal having a scale of circuitry for gain control use that is small and being of simple construction that is capable of obtaining a remarkably stable range of gain control and capable of reducing power consumption.